This invention relates to eddy current sensors, and particularly to eddy current sensors used for measuring various parameters of moving, electrically conductive objects, e.g. turbine blades, impellers, etc.
Eddy current sensors are known and used in a variety of applications. The present invention was developed in connection with the design of gas turbines and is described in connection therewith. The inventive sensors, however, have utility in other applications.
U.S. Pat. No. 4,847,556 (Jul. 11, '89) and 4,967,153 (Oct. 30, '90) both to Langley, disclose eddy current sensors for detecting various parameters, e.g. blade tip clearance, speed and transit time, of rotating blades of turbomachinery. The detected information is used for monitoring the performance and condition of the machinery.
The eddy current sensors disclosed in the patents (the subject matter of which is incorporated herein by reference) comprise a generally U-shaped member including two parallel legs each comprising a permanent magnet joined by a transverse flux bridge. One magnet has its North pole adjoining the flux bridge, and the other magnet has its South pole adjoining the bridge. This arrangement produces a static magnetic field bridging the space between the free ends of the magnets and spreading radially away therefrom. Two separate coils, connected in series, are disposed one each around each of the two permanent magnets, and the two series connected coils are connected to a signal processing circuit.
In the absence of any moving electrically conductive object within the static magnetic field produced by the two magnets, the static field remains undisturbed and no voltages are produced in the magnet mounted coils. However, when an electrically conductive object, e.g., the rotating blade of a turbine, passes through the static field of the two magnets, eddy currents are generated within the conductive object. The eddy currents themselves generate magnetic fields, and as these eddy current produced magnetic fields interact with the permanent magnet field, disturbances occur in the permanent magnet field which induce signal voltages in the two series connected coils mounted on the permanent magnets. Analysis of the induced coil voltages, as described in the patents, provides various information about the moving object, e.g., the speed of the object, its minimum distance from the sensor and the time of its closest passage by the sensor. As described in the patents, such information is useful for monitoring the operating characteristics of turbomachinery.
The present invention provides eddy current sensors having utility for generating information similarly as in the aforecited patents but having certain advantages over the sensors disclosed in the above-cited patents. These advantages are described hereinafter.